Processes for the removal of acid gases from hydrocarbon feedstreams have been widely practiced in the hydrocarbon processing industry. Typical acid gas components removed by such processes include CO.sub.2, H.sub.2 S, COS, SO.sub.2, SO.sub.3, CS.sub.2, HCN and oxygen and sulfur derivatives of C.sub.1 -C.sub.4 hydrocarbons, e.g., mercaptans. In such processes the gases are usually contacted with an absorption solution which is typically an aqueous alkaline solution that has capacity for the acid gases.
One common absorption solution is known as a "hot potash" solution. Typically, in a process utilizing a hot potash solution, a small amount of an amine is included as an activator for the aqueous base which typically contains an alkali metal salt or hydroxide, such as potassium carbonate, as the primary absorbing component in the solution. Processes which utilize such solutions are often used for the bulk removal of an acid gas such as CO.sub.2. Other typical absorption solutions are those which utilize an amine, such as an alkanolamine, as the primary absorbing component with little or no alkali metal salt or hydroxide.
In a typical absorption process for removal of acid gases, a regenerable aqueous alkaline scrubbing solution is continuously circulated between an absorption zone where acid gases are absorbed and a regeneration zone where they are desorbed usually by steam-stripping or flashing or both. There are a number of patents which describe absorption solvents and processes, some of which are described below.
In U.S. Pat. No. 3,856,921, issued to Shrier et al., there is disclosed a process for removal of acid gases from fluids by use of a basic salt of an alkali or alkaline earth metal and an amine selected from the group consisting of 2-methylaminoethanol, 2-ethylaminoethanol, morpholine, pyrrolidine and derivatives thereof.
U.S. Pat. No. 3,642,430, issued to Benson, discloses processes for removing CO.sub.2 and H.sub.2 S from gaseous mixtures by alkaline scrubbing processes wherein at least two separate regeneration zones are provided. Various alkanolamines, such as monoethanolamine, diethanolamine and aminoacids are described as activators in the absorption solution.
U.S. Pat. No. 4,112,050, issued to Sartori et al., discloses a process whereby carbon dioxide-containing acidic gases are removed from a normally gaseous mixture by a process comprising contacting the normally gaseous mixture with an aqueous solution comprising: (a) a basic alkali metal salt or hydroxide and (b) an activator for the basic salt or alkali metal hydroxide comprising at least one sterically hindered amine.
U.S. Pat. No. 4,336,233, issued to Appl et al., discloses a process wherein an aqueous solution containing piperazine is used as a washing agent for removing impurities such as H.sub.2 S, CO.sub.2 and COS from gases. The patent discloses that piperazine can also be used together with certain physical or chemical solvents to accelerate the absorption rate of H.sub.2 S, CO.sub.2 and COS.
In processes such as described above, the presence of carbonyl sulfide in the feedstream can present a problem. In general, absorption solutions such as described above do not have a high capacity for carbonyl sulfide and thus, carbonyl sulfide often comprises a portion of the acid gas impurities left in the product stream. Although high solution circulation rates can be employed to remove carbonyl sulfide by absorption with absorption solutions, the cost of constructing and operating such absorption systems can be prohibitive. Accordingly, combined absorption/adsorption processes have been proposed.
U.S. Pat. No. 4,717,552, issued to Carnell et al., discloses a process wherein the desulfurization of natural gas is effected by using a bed of particulate absorbent comprising zinc oxide at below 30.degree. C. before molecular sieve drying of the gas. The patent is directed to removing both water and hydrogen sulfide from a natural gas feedstream that also comprises carbon dioxide. It is known in the art that carbon dioxide and hydrogen sulfide can react to form carbonyl sulfide and that the reaction is reversible. The patent discloses that by removing the hydrogen sulfide in the zinc oxide bed prior to the molecular sieve drying stage, little or no carbonyl sulfide is formed during passage of the gas through the molecular sieve. The patent discloses at column 1, lines 39-46 that:
"While carbonyl sulfide can be removed, e.g., by an amine washing step or by absorption using a bed of a suitable absorbent material such as zinc oxide, amine washing steps are often undesirable as they also effect removal of carbon dioxide, which removal is often undesirable on economic grounds, while absorbent beds generally have only a poor capacity for carbonyl sulfide at low absorption temperatures."
U.S. Pat. No. 4,957,715, issued to Grover et al., relates to the purification of hydrocarbon containing gaseous mixtures which includes both carbon dioxide and one or more sulfur-containing compounds such as hydrogen sulfide, alkyl mercaptans, carbonyl sulfide and the like. The patent discloses contacting a feed gas with an absorbent capable of removing hydrogen sulfide and alkyl mercaptans and then contacting the effluent obtained therefrom with a liquid medium, e.g., an aqueous alkanolamine solution capable of removing carbon dioxide, hydrogen sulfide and alkyl mercaptans to produce a product gas having reduced concentrations of carbon dioxide, hydrogen sulfide and alkyl mercaptans. In the above-identified patent, it is disclosed that at least a portion and preferably substantially all of the carbonyl sulfide present in the effluent gas from the adsorber is converted to hydrogen sulfide in the absorption column and absorbed by the liquid medium.
U.S. Pat. No. 3,864,460, issued to Connell, discloses a process for removing hydrogen sulfide from natural or refinery gas streams by absorption with an alkaline liquid. The absorbed hydrogen sulfide is partially converted to sulfur in a Claus plant and the residual hydrogen sulfide and the tail gas from the Claus plant are concentrated by adsorption and recycled into the absorption stage at a higher concentration such that all of the hydrogen sulfide is removed from the gas stream as sulfur. The patent discloses that aqueous solutions of diethanolamine are preferred as the alkaline liquid when carbonyl sulfide is present in the contaminated gas.
Generally, the process methods described above have been effective for removing acid gases from hydrocarbon streams such as natural gas. As noted above, the processes that utilize a single liquid absorption solution to remove all of the acid gases often cannot be operated in an efficient manner with respect to each of the individual components removed, i.e., higher solvent circulation rates or more highly purified solvent may be required to remove components such as alkyl mercaptans and carbonyl sulfide than is required for the bulk removal of carbon dioxide, for example. Moreover, the processes that utilize an adsorption zone using a solid adsorbent for removing selected components and which additionally employ an absorption zone using a liquid absorbent for other components typically require separate regeneration schemes for the solid adsorbent and the liquid absorbent.
Accordingly, processes are sought for the removal of carbonyl sulfide as well as other acid gases such as hydrogen sulfide and carbon dioxide from gaseous and liquid hydrocarbon feedstreams. Moreover, processes are sought which can provide for the conversion of carbonyl sulfide to form carbon dioxide and hydrogen sulfide and the subsequent absorption of the more readily absorbable components in an absorption zone. In addition, processes are sought which can provide a highly integrated process that utilizes a common regeneration scheme.